A 3-dimensional (3-D) or volume x-ray image can be of value for diagnosis and treatment of teeth and supporting structures. A volume x-ray image for this purpose is formed by combining image data from two or more individual 2-D projection images, obtained within a short time of each other and with a well-defined angular and positional geometry between each projection image and the subject tooth and between each projection image and the other projection images. Cone-Beam Computerized Tomography (CBCT) is one established method for obtaining a volume image of dental structures from multiple projection images. In CBCT imaging, an image detector and a radiation source orbit a subject and obtain a series of x-ray projection images at small angular increments. The information obtained is then used to synthesize a volume image that faithfully represents the imaged subject to within the available resolution of the system, so that the volume image that is formed can then be viewed from any number of angles. Commercially available CBCT apparatus for dental applications include the Kodak 9500 Cone Beam 3D System from Carestream Health Inc., Rochester, N.Y.
While CBCT imaging is a powerful diagnostic tool, however, there can be cases where, even though volume imaging is beneficial, the full-fledged capability of CBCT imaging is not needed. This has been acknowledged, for example, in disclosures of U.S. Patent Application Publication No. 2007/0127801 entitled “Method for Limited Angle Tomography” by Kalke and U.S. Pat. No. 7,269,241 entitled “Method and Arrangement for Medical X-ray Imaging and Reconstruction from Sparse Data” to Siltanen et al. For some types of volume imaging, such as for use in guiding implant placement, for example, a rudimentary volume imaging capability would be useful. Volume imaging can also help to avoid superposition anomalies between adjacent dental structures. For uses such as these, numerous x-ray projection images, such as those provided from a CBCT system would not be required. Instead, enough volume information can be obtained using a smaller number of x-ray images, provided a spatial coordinate reference between images is maintained.
As a general principle, it can be advantageous to obtain the minimum number of x-ray exposures needed in order to obtain the volume diagnostic data. A complete CBCT series of projection images over a 180 degree orbit employs higher cumulative radiation dosage than does a partial series that is either taken over a smaller range of angles or uses fewer projection images taken at increased relative angular increments. Thus, the methods taught in the '7801 Kalke and '241 Siltanen et al. disclosures can help to reduce patient exposure where full CBCT imaging is not needed.
CBCT systems used for this purpose have both advantages and drawbacks. CBCT equipment maintains the radiation source and sensor at the correct geometry relative to the subject so that an accurate 3-D volume representation is possible. Unless the angular and spatial positioning of source and detector are precisely controlled, any attempt at volume imaging is not likely to provide reliable and precise results. Cost and availability of CBCT equipment, however, are other factors to consider. The CBCT gantry for dental imaging is more expensive than conventional digital x-ray equipment and may not be readily available at a particular treatment site. Significantly, the CBCT system is not usable in an operative or treatment setting, with the patient seated for treatment in the dental chair. Instead, the CBCT image series is obtained in a separate environment that is specifically set up for that purpose.
Commonly assigned patent application number PCT/FR10/00370 entitled “ALIGNMENT APPARATUS FOR DENTAL INTRAORAL RADIOGRAPHY” by Inglese et al., incorporated herein by reference, relates to obtaining suitable alignment between an x-ray source and detector for intraoral radiography, to enable intraoral imaging at an established alignment position. It would be advantageous to be able to use the alignment utilities described therein to help facilitate some level of volume imaging, without the disadvantages of requiring separate CBCT imaging equipment.
Thus, it can be seen that it would be advantageous to provide a method and apparatus for intraoral imaging that allows image data to be obtained that has the correct geometry and is usable for providing a volume image that can be used for some types of diagnosis and treatment, but does not require a CBCT system.